Why does my ulna in my forearm hurt when doing bicep curls with a dumbell?

Technique issue: Most commonly in biceps curls the forearm flexors also are used, which can create an overuse of these muscles. Also, the extensors that stabilize the flexors to maintain control during the curl may be strain with heavy weights or excessive repetitions. In rare cases stress fractures can occur.

Forearm and wrist: position has a significant effect on stresses to those regions when doing a bikes curl. It depends upon what portion of the wrist or forearm ones dressing BUT oversupination or rotated with palm too far up , may put a lot of stress on the ulna at the wrist or mid forearm. If it is only positional, thenone should modify that postion. If it is all the time one may need an evaluation for an injury